Intermittent Spark problem

[Jay2903]

Hey guys

I was wondering if anyone knows what could cause an intermittent spark like this.

Battery fully charged
Tried different plugs
Plug gaps correct
Points gaps correct
Timing corrrect

Points, condenser, plugs, battery, ignition coils, Ht lead & plug caps replaced recently.

Any help greatly appreciated
Cheers

[Easy-X]

Looks like you've covered most things. Check all earth points for corrosion and loose connections. Also the stator and associated wiring for damage.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[WD Forte]

What bike?
y u no tell us
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bikers smell of wee

[Riejufixing]

[mentalboy]

Is that an older kwak?

Have a giggle at this and then check your side stand switch...
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Make mine a Corona.

[Jay2903]

Cheers for the help guys. Completely forgot I posted on here. It was the aftermarket ignition coils. As soon as I switched back to the originals the spark was consistent and noticibly stronger.

[Jay2903]

I thought I had it but no.

I’m not sure if i’m missing something blindingly obvious but when I test the plugs, if I have them near the engine but not touching I can see a decent spark whereas if I have them directly touching the engine the spark is either really small or not at all. I’ve tried a few different plugs and this is happening on both sides.


Any ideas ?

[Easy-X]

Eh? To get a spark you need to complete a circuit. If you're not touching the plug to the engine (or any bare metal on the frame, hence "frame ground") then what is the other half of the plug connected to?!
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[jaffa90]

For a start you no answer questions.
Did you check all earths as in points to engine and engine to frame if needed?
What about the plug caps?

[Jay2903]

[Jay2903]

[Riejufixing]

Hang on, have you tried new sparking-plugs, or ones out of an old box of used ones?

[jaffa90]

[mentalboy]

[Tankie]

Go and check the earthing between the frame and engine, and while you are doing that remake the existing earthing points
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Always learning, everyday is a school day

[GettinBetter]

It's possibly tracking somewhere, from plug cap to chassis, or from the coil end, best spotted in the dark. Usually spraying WD40 on or around the suspected areas, will stop it temporarily... and prove the diagnosis (or not). meticulously cleansing, or renewal of parts will cure it.

UPDATE:
The system 'tracks' because the spark takes the path of least resistance, if you're using resistive HT cable and suppression caps, it doesn't take much dirt to create a lower resistive path, and if it intermittently sparks outside the cylinder, it usually fails altogether inside the cylinder.
It's easy to prove & easy to fix.

[Teflon-Mike]

Eliminate variables.
Who knows is that spark is intermittemt, or that the plug is just bouncing off the rocker cover when the engine cranked....
CAR JUMP LEAD!!! clamp the plug in croc-clip one end you know you got a good electrical contact. Other end croc-clip to a known Good earth.. preferably not a bit of frame or engine that's pained or laquered.
Then try again, and is the spark more consistent? And DO you actualkly have an intermittent spark?
If points system... yeah.. ergh! But the condensor was/is always my most likely usual suspect... and fact you say that its already been changed dont so a lot for my confidence... what was it changed with, and even a band new item, I would have doubts over how long it had been sat, drying out, on a shelf for.. especially for a more obscure model of old classic.
Old 'trick' was to get a new condenser from the mororfactors, and something common and hopefuly cheap, that comes out of a box that gets refreshed pretty often, the common as muck item for a Mini/Morris/Lucas distributor used to score well here; THEN isolate the OE connenser in the mag, and couple the car condenser between the coil input and a known good earth. Makes the same circuit, and does the same job, soking volts when points open, but car typr condenser usually has a little extra capacity, and not stuck against the crank case isn't having its electrolyte boiled dry or di-electric tissue cooked by engine heat. Plus better odds that new car type condener is closer to 'fresh' and not been sat on a store room shelf for forty years.
Whilst in there, I'd check the state of the points, and again, they maybe shop new, but how fresh are they? Contacts could be rusty as feck by now, or have been 'protected' with grease/oil thats now a nice layer of soot or crud. So clean and gap critically, and re-check timing.
Beyond that... first question is DO you actually have a miscellaneous spark?Second Is everything you think you have checked/eliminated really checked/eliminated... SO, what is the 'real' problem? Wont the engine start? Hold idle revs? Take load? What? And IS the ignition system a likely suspect in that?
Its an old engine, has it been rebuilt? Has it burned off assembly oil? Are the valve clearances correct? Does it have good compression etc.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Jay2903]

Cheers for the help guys

Where I’m at with it now. Whilst checking spark the electric starter stopped working, it was spinning but not turning the engine. I figured I’d look into it after I sorted the spark problem and used the kick instead.
It looked to have a consistent spark. I started it, it ran for a few seconds then stopped and I went to kick it again and it would budge. The key that holds the rotor onto the crankshaft had sheared again. This time it ruined the seat in the rotor.

Basically what I’m asking is.
What could be causing these keys to shear

2nd thing - what is the spec resistance for a xs500 plug cap. I can’t find it in the manual or online, I tried measuring the original one but I couldn’t get a reading so I took it apart and it was missing it’s diode, the spring was just bouncing around in it. I’m surprised it works at all. The replacement caps are 5k ohms.

[jaffa90]

What does """" I went to kick it again and it would budge. """" mean?
Also """"""The key that holds the rotor onto the crankshaft had sheared again. This time it ruined the seat in the rotor.""""""" any pics.

Rotor? Are you talking the sprung loaded points cam drive?

Also fit a non resistor / suppressor plug cap.

[mentalboy]

[Jay2903]

[Teflon-Mike]

The Montesa motors, especially the Cota trials versions with a heavier mag-rotor for slow-running were notorious for sheared keys; I've lost about half a dozen over the years. When 'new' it was usually caused by the rotor not being so carefuly fitted. When old/Resto'd, it was a similar looseness, usually caused by a little surface rust on the rotor and or crank taper, and or looser worn rotor nut.

On resto, the advice was to use a little valve lapping past on the crank-shaft, and spend indeterminate hours working the rotor back and forth on the crank taper to a) get the surfaces properly clean of any surface rust and or crud' b) clean thoroughly with thinners (esp after lapping with valve past) c) To critically fit the rotor onto the crank, which meant using a copper mallet to actually get the rotor to interference fit to the crank taper, rather than merely relying on the pressure made by the, probably worn, rotor nut.

If that's any ideas for you.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[mentalboy]

How's this for a larf, you tell us what machine we're guessing about here and somebody familiar with that model might have some ideas.

If it's a trials then I haven't the foggiest - I know it's definitely not an old Thumper as they didn't have a leccy start and it can't be a mkIII Commando with the nasty Polish starter that never worked.
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Make mine a Corona.

[Jay2903]

[Jay2903]